Ink-jet priming is a non-impact priming process in which droplets of ink are deposited on print media, such as paper, transparency film, or textiles. Low cost and high quality of the output, combined with relatively noise-free operation, have made ink-jet printers a popular alternative to other types of printers used with computers. Essentially, ink-jet printing involves the ejection of fine droplets of ink onto print media in response to electrical signals generated by a microprocessor.
There are two basic means currently available for achieving ink droplet ejection in ink-jet printing: thermally and piezoelectrically. In thermal ink-jet printing, the energy for drop ejection is generated by electrically heated resistor elements, which heat up rapidly in response to electrical signals from a microprocessor to create a vapor bubble, resulting in the expulsion of ink through nozzles associated with the resistor elements. In piezoelectric ink-jet printing, the ink droplets are ejected due to the vibrations of piezoelectric crystals, again, in response to electrical signals generated by the microprocessor. The ejection of ink droplets in a particular order forms alphanumeric characters, area fills, and other patterns on the print medium.
Ink-jet inks are mostly available as dye-based compositions. However, a very limited number of pigment-based inks are also available. Indeed, only two black pigment-based inks are commercially available at the present time; no color pigment-based inks are commercially available. Pigments do offer two very desirable properties: waterfastness and lightfastness. However, they have not found extensive use in ink-jet ink compositions partly due to their natural tendency to agglomerate in aqueous (hydrophilic) media as well as their lack of uniform size distribution. Difficulties in unheated bleed control constitute another reason pigment-based ink-jet inks are not extensively used.
The agglomeration of pigment particles and their size variability can result in poor print quality and reliability. In response to these problems, technology has been developed to encapsulate pigments in transparent polymer materials to render the pigment particles passive, thereby eliminating agglomeration. Moreover, the process of encapsulating the pigment particles also ensures a narrow size distribution thereof, so that variability in the thickness of the printed ink is reduced. The technology of encapsulating pigment particles is described in an article entitled "Color Pigment Encapsulation", written by Robert Lustenader (Ink World, January/February, 1995, pages 74-75). The technology of dispersing pigments in solvents with hydrophobic dispersants is also readily available. Surface modifications not involving either encapsulants or dispersants that render the pigment surface appropriate for suspension in solvents are also known.
While the agglomeration and size distribution problems associated with pigments have been addressed, the inherent waterfastness of the pigments has meanwhile often been sacrificed in the formulation of pigment-based ink compositions. More specifically, pigments are typically dispersed in aqueous media to form ink-jet ink compositions using water-soluble dispersants, thereby often losing their inherent waterfastness in the process. Thus, by dispersing pigments in aqueous media, one of the most advantageous features of pigments can be destroyed. Moreover, the dispersion of pigments in aqueous media renders pigment-based ink compositions subject to bleed--a problem that has not been overcome to date.
With regard to the problem of bleed, pigment-based inks formulated as dispersions in aqueous media exhibit bleed when printed adjacent to other aqueous ink compositions, whether dye-based or pigment-based. The term "bleed", as used herein, is defined to be the invasion of one color into another, as evidenced by a ragged border therebetween. Bleed occurs as colors mix both on the surface of the paper substrate as well as within the substrate itself. Bleed is particularly problematic in ink-jet color printing given that aqueous yellow, cyan, and magenta ink compositions are printed in various proportions and combinations adjacent to one another and to black ink. To achieve superior print quality, it is necessary to have borders between colors that are bleed-free.
Various solutions to the problem of black to color and color to color bleed have been proffered. Some solutions involve changing the ink environment to reduce bleed. For instance, heated platens and other heat sources, along with specially-formulated paper, have been employed to reduce bleed. However, heated platens add cost to the printer, and specially formulated paper is more expensive than "plain" paper. Thus, using external paraphernalia to reduce bleed in ink-jet color printing is generally not cost effective.
Other proposed solutions involve changing the composition of an ink-jet ink to reduce bleed. For example, surfactants have been effectively used to reduce bleed in dye-based ink formulations; see, e.g., U.S. Pat. No. 5,106,416 entitled "Bleed Alleviation Using Zwitterionic Surfactants and Cationic Dyes", issued to John Moffatt et al; U.S. Pat. No. 5,116,409 entitled "Bleed Alleviation in Ink-Jet Inks", issued to John Moffatt; and U.S. Pat. No. 5,133,803 entitled "High Molecular Weight Colloids Which Control Bleed", issued to John Moffatt, all assigned to the same assignee as the present application. However, surfactants increase the penetration rate of the ink into the paper, which may also result in the reduction of edge acuity. Other solutions specific to dye-based ink compositions, disclosed in patents assigned to the present assignee, are found in U.S. Pat. No. 5,198,023, entitled "Cationic Dyes with Added Multi-Valent Cations to Reduce Bleed in Thermal Ink-Jet Inks", issued to John Stoffel, and U.S. Pat. No. 5,181,045, entitled "Bleed Alleviation Using pH-Sensitive Dyes", issued to James Shields et al, both assigned to the same assignee as the present application.
While the problem of black to color and color to color bleed has been the subject of much study, none of the solutions offered are applicable to pigment-based ink compositions, save for the use of heated platens that are not cost effective. For example, surfactants, if added in concentrations similar to those used in dye-based formulations to achieve bleed control, would destabilize the pigment dispersions made with water soluble dispersants. This is because the dispersants, themselves being surface active compounds, would be competitively displaced by the added surfactants that compete to attach to the pigment surface.
Additionally, ink formulations with improved dry times are always sought in ink-jet printing in order to gain in throughput.
Accordingly, a need exists for pigment-based inks exhibiting reduced bleed and improved dry times while retaining the inherent waterfastness of the pigment itself.